There is no obvious way to use a third-party language server with the Xcode editor.
This would seemingly require using an editor extension to trigger communication with the external language server on every keystroke, automatically (at least for code completions and syntax highlighting) - which isn't possible.
It isn't obvious, either, how one would make Xcode use a custom version of SourceKit-LSP, even if one could hack it for other languages.
Am I wrong about this?
custom xclangspecs work well enough for syntax highlighting and code folding, but LSP's other functionality is sorely missed outside of C, Objective-C and Swift.